The present invention relates to communications networks. More particularly, and not by way of limitation, the present invention is directed to a system and method of downloading data to an unsynchronized User Equipment (UE) in a telecommunications network. A key advantage in “Long Term Evolution (LTE) Advanced” is a decrease in user plane and control plane latency or delay. Third Generation Partnership Project (3GPP) is specified as a goal that special attention should be given in situations where the UE does not have a valid scheduling assignment and/or needs to synchronize to the network.
FIG. 1 illustrates a simplified block diagram of a UMTS network 100 that comprises a 3rd Generation (3G) network referred to as a core network 102 and a UMTS Terrestrial Radio Access Network (UTRAN) 104. The UTRAN comprises a plurality of Radio Networks Controllers (RNCs) 106. There is a plurality of RNCs performing various roles. Each RNC is connected to a set of base stations. A base station is often called an Evolved Node B (eNB) 108. Each eNB 108 is responsible for communication with one or more UEs 110 within a given geographical cell 112. The serving RNC is responsible for routing user and signaling data between an eNB and the core network.
For a UE to transmit data in an Uplink (UL), the UE must be time synchronized to the network, otherwise the UE's transmissions may cause interference to other UE transmissions as well as receiving interference from other UEs. Synchronization is achieved by the eNB sending time alignment (TA) commands to the UE such that the UE can adjust its transmission timing. When the UE has not received any TA command for an extended period of time, a time alignment timer in the UE expires and the UE is considered not to be synchronized anymore. In order for the UE to start sending data when it is not synchronized, the UE must first perform a random access procedure, which is followed by a TA command sent by the eNB. When the UE has received the TA command (and a grant that allocates radio resources), the UE can then transmit its data in an uplink.
In principle, a UE can receive data in a Downlink (DL) without being time synchronized to the network. However, in this situation the UE cannot transmit any Hybrid Automatic Repeat Request (HARQ) feedback, such as Acknowledgements/Negative Acknowledgements (ACK/NACKs) since these messages require the UE to be synchronized. Thus, the normal operation for transmitting data to an unsynchronized UE involves several steps. FIG. 2 is a signaling diagram illustrating a typical sequence to transmit data to an unsynchronized UE 110. The network 100 first orders the UE 110 to perform a Random Access (RA) by sending a request 200 through the eNB 108 on a Physical Downlink Control Channel (PDCCH). The UE responds to the request by performing a RA procedure 202 which involves sending a random access preamble to the eNB. The request on the PDCCH includes an indication of which preamble the UE shall use when making the RA to avoid the need for a contention resolution handshake before DL transmission. The eNB can also choose not to indicate any dedicated preamble, in which case a contention resolution procedure is performed as part of the RA (which is not shown in FIG. 2). Next, the eNB responds by transmitting a TA command 204 to adjust the UE's transmission timing. After the TA command has been sent, the eNB can then start transmitting downlink data 206.
To minimize the user plane delay for unsynchronized UEs, it is possible to transmit data to these UEs, as long as HARQ is not used. However, this has the clear drawback that transmission losses cannot be recovered by HARQ. Instead, the losses must be recovered by Radio Link Control (RLC), which again increases the delay. One existing solution is to repeat the transmission blindly for a number of times to increase the reliability (e.g., the data can be transmitted 3 times). However, this procedure is inefficient since oftentimes only one or two transmissions are needed, or in other instances, sometimes three transmissions would not be enough. FIG. 3 is a signaling diagram of a blind HARQ transmission procedure to an unsynchronized UE. As depicted in FIG. 3, the eNB 109 sends an initial transmission 210 of data to the UE 110. Next, the eNB sends a first retransmission 212 of data to the UE. A second retransmission is then sent from the eNB to the UE.